In recent years, an energy generation system that generates energy by making use of a forward osmosis membrane has come to be used. The forward osmosis membrane has a configuration illustrated in FIG. 11 and FIG. 12, in which, when salt water is supplied to a salt water passage of the forward osmosis membrane and fresh water is supplied to a fresh water passage of the forward osmosis membrane, fresh water permeates the salt water passage from the fresh water passage. FIG. 11 is a schematic diagram illustrating the configuration of the forward osmosis membrane, and FIG. 12 is a schematic diagram for describing an osmosis phenomenon in the forward osmosis membrane.
As illustrated in FIG. 11, a forward osmosis membrane 100 includes: a salt water passage 102 to which salt water is supplied and through which salt water passes; and a fresh water passage 101 to which fresh water is supplied and through which fresh water passes. Salt water is supplied to the salt water passage 102. Likewise, fresh water is supplied to the fresh water passage 101. The forward osmosis membrane 100 further includes a separation membrane 103. As indicated by an arrow in FIG. 12, fresh water permeates the salt water passage 102 from the fresh water passage 101 via the separation membrane 103.
As a result of this osmosis phenomenon, on the salt water passage 102 side, the permeating fresh water is added to the supplied salt water, whereby mixed water is produced. The pressure on the salt water passage 102 side is higher than that on the fresh water passage 101 side. The mixed water has a water amount increased by the fresh water while maintaining the high pressure of the salt water.
On the other hand, on the fresh water passage 101 side, fresh water that has not permeated the salt water passage 102 passes through the fresh water passage 101 as it is, and flows out as non-permeating water. This non-permeating water is only required to be discharged.
The mixed water has the same high pressure as that of the supplied salt water. The permeation of fresh water causes an increase in the amount of water having a high pressure, thereby allowing energy to be generated. For example, the mixed water having a relatively increased water amount allows the generation of electric power and the operation of machines. In other words, the mixed water allows the generation of energy.
At this time, the water amount of the mixed water is larger than that of salt water supplied to the salt water passage 102. This differential in water amount leads to energy generated by the mixed water produced via the forward osmosis membrane 100. In other words, energy generated based on this differential in water amount is energy newly generated by the mixed water.
With the mixed water thus-produced by an osmosis phenomenon, the forward osmosis membrane 100 allows the generation of energy higher than energy in the case where only salt water is used.
The energy generated by the mixed water in the forward osmosis membrane 100 can be put to various uses. For example, the use of the mixed water for the rotation of a turbine allows electric power to be generated. In other words, a differential in energy caused by a differential in the amount of flow between salt water before supplied and the mixed water is energy generated in the forward osmosis membrane 100, and, with thus-generated energy, electric power can be newly generated.
Alternatively, when the mixed water is used for operating a pump or a machine, a differential in the amount of flow similarly causes a difference in the amount of energy, thereby yielding a new operation. In other words, an energy generation system that uses the forward osmosis membrane 100 is capable of generating a differential energy that corresponds to, for example, the generation of electric power or the operation of machines.
The forward osmosis membrane 100 can be thus used as an energy generation device. As a result, an energy generation system that uses the forward osmosis membrane 100 has been proposed (for example, refer to Patent Literature 1).
On the other hand, as described above, energy generated by the energy generation system using the forward osmosis membrane 100 is based on a differential in the amount of flow between salt water before supplied to the forward osmosis membrane 100 and the mixed water. This is because, without the differential in the amount of flow, the differential between the amount of energy owing to the salt water before supplied and the amount of energy owing to the mixed water is not made, and hence, it cannot be said that energy is newly generated.
Furthermore, the energy generation system that uses the forward osmosis membrane 100 needs energy to be used related to, for example, the supply of salt water or fresh water or the discharge of the mixed water and non-permeating water. In other words, the net amount of energy generated by the energy generation system is an amount obtained by subtracting energy used from energy generated by the mixed water. Even in the case where the amount of energy generated by the mixed water is large, when the energy used is too large, the energy generation system that uses the forward osmosis membrane 100 becomes useless.